1. Field of the Invention
The present invention is directed in general to a magnetic resonance tomography apparatus and to a method wherein a non-linear gradient field is superimposed on a basic magnetic field transversely to a coding direction and a location coding ensues by moving a subject in this coding direction. In particular, the present invention is directed to a magnetic resonance tomography apparatus and to a method wherein it is possible to acquire absolute location information from a relative motion of a subject by a specific distance in a coding direction.
2. Description of the Prior Art
Magnetic resonance tomography is a method for medical diagnostics that makes it possible to display structures of the human body overall and in detail. Nuclear spins of the nuclei are aligned in a region under examination in a subject due to a strong, externally applied magnetic field. Statistically distributed, the nuclear spins align in specific energy levels. When energy is supplied by means of a radio-frequency pulse at a resonant frequency that is material-specific, the nuclear spins assume different energy levels. After the radio-frequency pulse is deactivated, the return onto the original energy level can be measured in the external magnetic field by receiving the energy emitted at the resonant frequency.
It is decisive for magnetic resonance tomography that this signal of the nuclear spins on the resonant frequency be able to be allocated to a location in the subject. To this end, phase coding or frequency coding is employed. A number of detailed, even combined applications are known for both methods in order to achieve a optimally precise location encoding under certain conditions, either in a specific volume element or a rod-shaped volume or a surface.
Frequency coding is based on the use of a gradient field that linearly decreases or rises in a coding direction that is superimposed on the basic magnetic field by which the nuclear spins are aligned. Since the resonant frequency of the nuclear spins—more precisely, the energy levels of the nuclear spins—exhibits a dependency on the intensity of the magnetic field, selection of a slice can ensue as a result. This slice is perpendicular to the direction in which the magnetic field decreases or rises linearly. The resonant frequency of a specific substance is linearly dependent via the constant γ—the gyromagnetic constant—on the magnetic field at the location of the nucleus. In magnetic resonance tomography, an examination usually is undertaken on the basis of the nuclear spins of hydrogen in order to obtain an image of the tissue distribution of a body segment under examination by virtue of their distribution. When the gradient field is superimposed on the basic magnetic field, those nuclear spins that exhibit the resonant frequency that corresponds to the magnetic field in this layer, via the gyromagnetic constant, can be received only at a specific frequency.
Such a gradient field is generated, for example, by two coils that each generate a magnetic field, the two magnetic fields being oppositely directed. A magnetic field that essentially linearly rises or falls relative to a spatial direction arises in a volume between the two coils. It is desirable to be able to image an optimally large measurement volume in a nuclear magnetic resonance tomography apparatus, so that an optimally large region of a subject to be measured can be covered at one time. When such a very large field of view (FOV) is desired, however, serious image disturbances or artifacts occur because the magnetic field no longer falls or rises with enough linear exactitude. Quadratic field inhomogeneities occur that are known as concomitant fields or Maxwell terms. The order of magnitude of these quadratic inhomogeneities for a gradient field transverse to the basic magnetic field can be estimated for the direction of the basic magnetic field, as the coding direction, as being proportional to the square of the magnetic field strength of the superimposed gradient field divided by twice the basic magnetic field times the location in the square. Leaving the contribution of this non-homogeneous field component out of consideration is even less possible when stronger gradient fields are employed, for example greater then 10 mT/m, as well as in systems wherein a lower basic magnetic field, for example less than 0.5 Tesla is employed. The correction or avoidance of artifacts and other image errors that are based on these effects therefore is becoming increasingly important. This problem area is discussed in the article by Zhou, Tan and Bernstein, “Artifacts induced by concomitant magnetic field and in fast spin-echo imaging” in Magnetic Resonance Medicine (MRM) 40.582-591 (1998).
The second previously mentioned, basic location coding method—phase coding—is based on the effect that, when an excited condition of a nuclear spin is subjected to a specific magnetic field and resonates with the corresponding resonant frequency, the phases of the excitation states of the nuclear spins (initially in phase) diverge when they are subjected to different magnetic fields in the same time span, and thus temporarily exhibit different resonant frequencies. Typical phase coding methods are consequently based exposing a region of the field of view to a gradient magnetic field and, consequently, the phases diverge location-dependently. Typically, this method is multiply implemented with gradient magnetic fields of different intensities in standard phase coding sequences. In order to keep the overall measuring time short, it is desirable to keep the time during which the gradient magnetic field is activated short, since, as noted, multiple repetitions are necessary. As a result thereof, however, the inhomogeneities of the field that have been discussed above have an especially pronounced effect and can no longer be left out of consideration, particularly in the phase coding.
By combining the above-described, basic location coding methods—either successively or in different spatial directions—as well as with a preceding selective excitation of only specific nuclear spins in a specific volume element, for example an area, a large variety of known coding methods are available. Given a larger field of view, a negative effect in all of these methods arises from the aforementioned quadratic field terms of the magnetic field intensity. These have a pronounced effect as discussed above particularly in those modern nuclear magnetic resonance apparatus that operate with a low basic magnetic field and those operating with strong gradient magnetic fields.